An amplifier is one of circuit elements widely required in LSIs of any type. Two types of amplifiers are mainly known as a wide-band amplifier used to perform amplification or transmission of high-speed signals. One is an amplifier that utilizes an inductor peaking (a technique for realizing a widening of a band by using an inductor), and the other one is an amplifier that utilizes a wide-band characteristic of a transmission line. As a typical example of the amplifier that utilizes the wide-band characteristic of the transmission line, a distributed-constant-type amplifier can be cited.
FIG. 8 illustrates a first example of the amplifier. An amplifier AMP1 is formed by coupling a plurality of stages of unit circuits U1 to Un in order to obtain a desired gain by an entire amplifier or to obtain a current driving capability that is required in an output part. Input terminals Ia, Ib (gates of transistors T1a, T1b) of the unit circuit U1 are coupled to input terminals INP, INN of the amplifier AMP1. Output terminals Oa, Ob (terminals of inductors L2a, L2b) of the unit circuit U1 (U2 to Un−1) are coupled to input terminals Ia, Ib of the unit circuit U2 (U3 to Un). Output terminals Oa, Ob of the unit circuit Un are coupled to output terminals OUTP, OUTN of the amplifier AMP1. Bias terminals B (gates of transistors T2) of the unit circuits U1 to Un are coupled to a bias terminal BIAS of the amplifier AMP1. To the output terminals OUTP, OUTN of the amplifier AMP1, terminating resistors RTa, RTb are coupled.
The unit circuit Ui (i=1, 2, . . . , n) is a TRA (Triple-Resonance Amplifier) type differential amplification circuit formed by applying an inductor peaking to a resistance-load-type differential amplification circuit being the most common differential amplification circuit. More specifically, the unit circuit Ui is formed as a resistance-load-type differential amplification circuit formed of differential pair transistors T1a, T1b, a current source transistor T2, and load resistors R1a, R1b, in which peaking inductors L1a, L1b are inserted between the resistors R1a, R1b and a power line, and peaking inductors L2a, L2b are coupled to connection nodes of the transistors T1a, T1b and the resistors R1a, R1b. The inductor peaking is the most effective method for realizing a widening of a band of a differential amplification circuit, and in theory, it is possible to widen the band by about 3.5 times by applying the inductor peaking, when compared with a case where no inductor peaking is applied.
FIGS. 9A, 9B, and 9C illustrate problems in the TRA-type differential amplification circuit. Note that FIG. 9A illustrates an equivalent circuit of the unit circuit (TRA-type differential amplification circuit) in FIG. 8. FIG. 9B illustrates gain-frequency characteristics of the equivalent circuit in FIG. 9A. FIG. 9C illustrates a substantial part of the gain-frequency characteristics in FIG. 9B. In FIGS. 9B and 9C, a characteristic curve CV1 corresponds to a gain-frequency characteristic when capacitances C1, C2 in the equivalent circuit in FIG. 9A are the same, and a characteristic curve CV2 corresponds to a gain-frequency characteristic when the capacitance C2 is twice the capacitance C1 in the equivalent circuit in FIG. 9A.
A frequency ω2 at which a resonance of an LC circuit formed of the capacitors C1, C2, and an inductor Lc (a portion surrounded by a dotted line in the equivalent circuit in FIG. 9A) occurs is represented by an expression (1) by using a frequency ω1. Further, an output voltage Vout is represented by an expression (2) by using an input current Iin and a resistance R.ω2=√2×ω1  (1)Vout=√(3/2)×Iin×R  (2)
When the capacitances C1, C2 are the same, a gain at the frequency ω2 becomes higher than a DC gain by 1.8 dB, but, when the capacitance C2 is twice the capacitance C1, the gain at the frequency ω2 becomes lower than the DC gain by 6 dB. In other words, when the capacitances C1, C2 are the same, a gain becomes lower than the DC gain by 3 dB at a frequency ω4, but, when the capacitance C2 is twice the capacitance C1, the gain becomes lower than the DC gain by 3 dB at a frequency significantly lower than the frequency ω4.
In order to optimize inductors Lc, Lr in the equivalent circuit in FIG. 9A, the capacitances C1, C2 of both ends of the inductor Lc have to be the same as a prerequisite. For this reason, when there is a need to gradually enlarge a driving capability (transistor size) of the unit circuits U1 to Un toward a rear stage of the circuits in the amplifier AMP1 in FIG. 8, it is not possible to equalize capacitances of both ends of the inductors L2a, L2b in each of the unit circuits U1 to Un, resulting in that a desired increase in the band cannot be obtained, which is a problem. If such a limitation exists, a usability as an amplifier is significantly reduced.
FIG. 10 illustrates a second example of the amplifier. An amplifier AMP2 is a distributed-constant-type amplifier formed by including an input-side transmission line LNA, an output-side transmission line LNB, and unit circuits TU1 to TUn. An input terminal IN of the amplifier AMP2 is coupled to one end of the input-side transmission line LNA, and a terminating resistor RTA is coupled to the other end of the input-side transmission line LNA. A terminating resistor RTB1 is coupled to one end of the output-side transmission line LNB, and a terminating resistor RTB2 and an output terminal OUT of the amplifier AMP2 are coupled to the other end of the output-side transmission line LNB. A unit circuit TUi (i=1, 2, . . . , n) is formed of a transistor having a gate coupled to the input-side transmission line LNA, a source coupled to a ground line, and a drain coupled to the output-side transmission line LNB, and has no amplifying capability. For this reason, a plurality of numbers (n number) of unit circuits are coupled between the input-side transmission line LNA and the output-side transmission line LNB in order to obtain a desired gain by the entire amplifier.
In the distributed-constant-type amplifier with such a structure, the transmission line can be approximated by a ladder-type LC circuit, and a widening of the band realized by the transmission line can be expected. In a simple theory, by coupling n unit circuits, a gain of an amplifier is increased by n times, when compared with a case where one unit circuit is coupled. However, when the number of unit circuits is increased, a transmission line becomes long, resulting in that a loss in the transmission line becomes significant. Accordingly, a gain-frequency characteristic becomes the one in which a gain is gradually decreased from a low frequency side to a high frequency side, and it is difficult to realize a flatness of the gain-frequency characteristic. In the distributed-constant-type amplifier, a circuit design has to be made carefully by taking this point into consideration, otherwise the band is narrowed.
Note that as documents related to the embodiments, Japanese National Publication of International Patent Application No. 10-510970, S. Galal and B. Razavi, “40 Gb/s Amplifier and ESD Protection Circuit in 0.18 μm CMOS Technology,” ISSCC Dig. Tech. Papers, pp. 480-481, February 2004, and the like can be cited.
As described above, the first example has a problem that there is a limit in widening the band when there is a need to gradually enlarge the driving capability of the unit circuits toward the rear stage of the circuits, and the second example has a problem that it is difficult to secure the flatness of the gain-frequency characteristic.